Pumps for extracting oil from oil wells include elongated cylinders lowered into the well with a piston and piston rod connected through appropriate links to the surface. Valves are incorporated in the lower end of the cylinder and in the piston so that reciprocating movement of the piston in the cylinder will draw up oil to the surface. The piston rod connectors up to the surface terminate in what is called a polish rod extending through an appropriate seal from the upper end of the well. This polish rod is connected to a connector cable associated with a reciprocating member referred to as a horse head. The horse head is caused to oscillate up and down by an appropriate drive at the surface of the well.
Given fixed dimensions for the pump cylinder within the well, the rate of pumping of oil will depend on the overall length of the pump stroke and the number of strokes per unit time. Normally there is an optimum speed of operation for the pump. That is, the horse head type pumps best operate at a uniform given rate of reciprocation. Therefore, the only means for varying the rate at which oil is removed from the well is by varying the pump stroke.
It is to be appreciated that the effective pump stroke in removing oil in itself varies depending upon the depth of the well. For extremely deep wells, there is necessarily a "stretching" of all of the connecting rods between the polish rod and the pump piston at the bottom of the well. In other words, a certain part of the stroke is taken up in simply "stretching" the rods before actual movement of the lower end of the rod occurs. The same situation obtains on the downstroke of the pump.
In addition to the foregoing, after prolonged pumping of wells, the available oil at the bottom of the well may decrease or be more difficult to "suck" out of the formations. For this reason it may be desirable to utilize a shorter stroke to effectively decrease the rate of pumping of the oil to better match the situation in the formations.
Heretofore, the pump stroke of the horse head type rocking pumps has been varied by simply shifting the pivot point of the Pittman with respect to the crank arm, or by any other equivalent means shortening the crank arm which drives the Pittman which in turns rocks the walking beam carrying the horse head back and forth. Such an operation requires personnel to visit the field with appropriate tools and to shut down the pumping operation until appropriate adjustments have been completed. These adjustments are a time consuming operation and must be carried out every time. it appears desirable to change the pump stroke.